1. Field of the Invention
The present invention relates to a system and program for demoting tracks from cache.
2. Description of the Related Art
When data is successfully written to a hard disk drive, the drive returns a write complete message to the host system that initiated the write operation. However, if the read/write head of the hard disk drive is not operating properly, the disk drive may return a write complete without actually writing the data to the disk. In large enterprise storage systems, the disk drive may return a complete to a destage of updated data to the drives. If the read/write head does not write the data even though complete is returned, the data is lost and recovery may not be possible using error correction techniques or Redundant Array of Independent Disk (RAID) algorithms because the data was never written to the disk. This type of error is called a “dropped write” error. Further, once a read/write head starts dropping writes, typically all writes following the failed write will also be dropped.
Dropped write errors may corrupt the parity data because the parity data for the dropped write is inconsistent with the data on the drive, which does not include the dropped write. Subsequently calculated parity based on the block to which the dropped data should have been written would be corrupt because it is not calculated using the dropped data, thereby preventing recovery and reconstruction of the dropped data using the parity data.